Listing GMail delegates of users (Via Service Account)

Google Workspace Developers Q&A
1

I am trying to use this API call - https://developers.google.com/gmail/api/reference/rest/v1/users.settings.delegates/list

To list the delegates of users in Gmail.

I have created a Service Account, given it Domain Wide delegation, and created a js script to generate a JWT and access token.

The service account has the right scopes and everything, but…i’m still stuck. The error i’m getting is

{
  "error": {
    "code": 401,
    "message": "Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
    "errors": [
      {
        "message": "Invalid Credentials",
        "domain": "global",
        "reason": "authError",
        "location": "Authorization",
        "locationType": "header"
      }
    ],
    "status": "UNAUTHENTICATED"
  }
}

Does anyone have any experience in working with this delegate endpoint, and using a Service Account, as i’m pulling my hair out.

I know the access token only lasts 1hour (3600 secs).

The script i’m using to generate the JWT seems OK

So, any assistance would be appreciated.